Method of making electrical resistance elements and alloys therefor



Patented June 2, 1936 UNITED STATES",

PATENT OFFICE METHOD OF MAKING ELECTRICAL RESIST- ANCE ELEMENTS AND ALLOYS FOR THERE- James L. Thomas, Garrett Park, Md., assignor to the Government of the United States, represented by the Secretary of Commerce No Drawing.

Application November 22, 1934, Serial No. 754,310 r 1 Claim. (01. 201-76) (Granted under the act of March 3, 1883, as amended April 30, 1928; 3'10 0. G. 757) ways been encounterd in obtaining a wire whose resistance'remains stable or substantially constant. It is known that resistance changes with age and also with ordinary changes in laboratory temperature. The change which takes place with age may be due to chemical action of the air upon the surface of the wire, or to changes in the crystal arrangement therein.

Further, besides the stability, resistance alloys should have low temperature coeflici'ents of resistance in order that they may be readily measured to a high precision.

To the best of my knowledge, no alloy has been developed which is entirely free fromchanges in resistance with time or with temperature, but special alloys have been developed for which the Ichanges in resistance with temperature and time are comparatively small. Manganin, an alloy of copper, manganese and nickel is the alloy most generally used for this purpose at present, but this alloy does not possess the desired qualities to a sufliciently high degree.

I have conducted experiments with an alloy consisting mostly of gold with a small addition of chromium. Specifically, my research and tests have included a number of difierent alloys composed of gold with the addition of approximately from 1.5 to 5 percent of chromium by weight. As a result of the tests I have found the most desirable mixture is approximately 2.1 percent of chromium and the remainder gold. Moreover,

I have discovered that the addition of 1.5 to 5 percent of chromium to gold produces alloys having very small temperature coeificients of electrical resistance and that the temperature coefficient of the alloys depends upon the proportions of the two ingredients and the method of forming the finished article.

The first difliculty encountered was to discover a method ,by which the two ingredients could be properly alloyed, and after a number of tests, I found the best method is to melt the gold and chromium together, in graphite crucibles, under-a heavy coating of borax, the coating of borax being for the purpose of exclud- 5 ing air and thus preventing oxidation of the alloy. The crucibles are preferably heated by means of an induction furnace, and when a temperature of about 1200 C. is reached, the alloys are poured into the graphite molds, the 10 graphite crucibles and molds being preferable because graphite absorbsoxygen and will assist in the prevention of oxidation. While other methods of preparation would serve the purpose, such as alloying the metals in a vacuum or in an atmosphere of dry hydrogen, I prefer to use the one just described because of its simplicity and the good results obtained. The use of a vacuum or dry hydrogen atmosphere requires a complicated and expensive apparatus and does not render any 20 better, if as good, results as my preferred method.

After being molded, the ingots may be first hotforged into the form of. rods 5 or 6 mm. square. Then they are preferably cold-rolled and swaged to a diameter of 0.75 mm., after which they are drawn down to the required size through appropriate dies; such as sapphire dies. During the rolling and swaging the material may be annealed four or five times, but in my preferred method the wire is not annealed during the process of so drawing.

After the wire has been formed into a coil it is put through a baking process which has the effect of aging it artificially-that is to say, when I used the 2.1 percent chromium coils they were baked for several hours at a temperature of approximately 200 0., and after this baking the change in resistancawith' time, is much slower than for unbaked coils.

This baking has another effect besides that of 4 artificially aging the resistance of the coil. When the coils arebaked at a temperature of from C. to 500 C. the'temperature coefficients of resistance are decreased. The amount of this decrease in'temperature coefflcient was found to depend on both the baking temperature and length of .time the coils are baked. By properly choosing the temperature and time of baking it is possible to adjust the temperature coefficient to vzero for coils having a positive temperature co- 50 eflicient before baking. For example, an unbaked coil of the 2.1 percent chromium alloy had a temperature coefficient of parts per million per degree C. Its temperature coeflicient of resistance was reduced to zero by baking at 200 C. for about 5 hours.

The following table is illustrative of the temperature-resistance changes of a 10-ohm coil of the 2.1 percent chromium alloy baked at a temperature of 200 C. to bring its value to zero:

Temperature Resistance tial step in its artificial aging and has the advantage of doing away with, or at least substan-- tially reducing, the aging that would ordinarily take place as the element ages with time.

When manganin is used, in the construction of resistance standards, the coils are usually kept for a year or more in order to reach a reasonably stable state, but with my alloy and the hereindescribed process of making the coils, this aging is not necessary, and, obviously, is greatly preferable to the manganin.

As will be evident to those skilled in the art, my invention permits of various modifications of the method employed and of changes in percentage of elements in the alloy without departing from the spirit thereof or the scope of the appended claim.

What I claim is- The method of making an electrical resistance element which includes the steps of combining gold and chromium, the chromium being in the percentage of from 1 to 5 percent and the remainder being gold, forming the wire into wire strands and adjusting the temperature coeflicient of said resistance element to zero by baking said element in an atmosphere of air at a temperature of from C. to 500 C.

JAMES L. THOMAS. 

